Interleukin-15 (IL-15) is a vertebrate immune system modulating protein (cytokine) that stimulates the proliferation and differentiation of T-cells. In the clinical context, IL-15 is useful for the treatment of any of a variety of conditions such as, e.g., cancer. The ex-vivo manufacture of IL-15, however, can be problematic, and there is a need in the art for improved IL-15 products.